The present invention pertains low pressure airflow sensors. More particularly, it pertains to a sensor which can determine low pressure differentials.
Pressure sensors are utilized most commonly as pressure switches, turning on, as an example, electronic air cleaners when a pressure differential is determined. A pressure switch is disclosed in commonly owned, copending application entitled, "Differential Pressure Sensor Utilizing Opto-Reflective Sensor", Ser. No. 07/816,016, filed on this same date, and is hereby incorporated by reference. The switch compares the pressure downstream of the air cleaner to the pressure of the air surrounding the air cleaner. Usually, there is a return duct between the air cleaner and a grill to ambient air. When a return duct is present the pressure differential the airflow switch senses is the pressure differential between the pressure following the air filter and the ambient air pressure.
An example of a prior art pressure switch is U.S. Pat. No. 4,122,337, issued to Okuda et al. The pressure switch illustrated in Okuda et al. is a pressure electrical signal conversion means comprising a diaphragm assembly which is moved in accordance with the difference of pressure between the two compartment therein. Illumination means directs light in a band onto a photo-sensitive unit which includes two elements so disposed that when any particular band of the photo-sensitive area is illuminated, the illuminated area of one element is greater than that of the other. The photo-sensitive element produces an output proportional to the relative size of the illuminated areas of the elements. The photo-sensitive unit of an illumination means being moveable relative to one another in proportion to the movement of the diaphragm. Examples illustrate mounting a plate onto a diaphragm and preventing light from reaching a sensor through a shield until a specified pressure is reached. A second method shown illustrates attaching the sensor itself to the diaphragm, and determining the position of the diaphragm relative to where the light is reflected on the sensor.
As it is common for pressure sensors to have elements mounted upon the diaphragm in order to determine movement of the diaphragm and thereby determine the relative differential pressure, it is necessary to mount the airflow pressure sensor in a specific orientation. This is due to the weight of the material which must be added to the diaphragm. It is common to add materials to the diaphragm to either reflect an optical signal, to block an optical signal or to increase the effective area where optics are not used. The weight of this material will change the sensitivity of the diaphragm dependent upon what the orientation of the diaphragm is relative to the earth's gravitational forces. Therefore, it is necessary when utilizing a pressure sensor of this type to calibrate the sensor in a first orientation and mount that sensor in that same orientation or the pressure sensor will not be accurate. An example would be if a pressure sensor were calibrated in a first position, wherein the diaphragm moves in an up and down motion due to the differential pressure changes. When mounted for use, if the pressure sensor were mounted in a position where the diaphragm moved in a side to side motion relative to the earth's gravity, the material added would not effect the diaphragm in the same manner and accuracy would be lost. Therefore, when utilizing prior art pressure sensors, it is necessary that the pressure sensor be mounted .+-.10 degrees in the same orientation as it was when calibrated.
Applicant's invention proposes a compensation arm which allows the pressure sensor to be calibrated in a first position and mounted in any orientation which is necessary for use of the pressure sensor.